The combustion equipment of a typical gas turbine engine is required to operate efficiently over a wide range of conditions while at the same time producing minimal quantities of noxious emissions, particularly those of the oxides of nitrogen. This, unfortunately, presents certain problems in the design of suitable fuel injection devices for use as part of the combustion equipment. Thus the characteristics of a given fuel injector under light-up and low speed conditions are different to those under full power conditions. Consequently a fuel injector is often a compromise between two designs to enable it to operate under both of these conditions. This can result in combustion equipment which produces undesirably large amounts of the oxides of nitrogen, particularly when it is operating under one of these sets of conditions.
EP 0660038 describes one form of gas turbine engine fuel injector which is provided with two fuel supply ducts. Fuel is supplied through one supply duct under starting or low power conditions and through the other or through both fuel supply ducts under high power conditions. The fuel from both ducts is mixed with air in such a way that efficient, low emission combustion takes place under a wide range of engine operating conditions.
GB 2010408 describes a somewhat different approach to the reduction of noxious emissions in which a gas turbine engine annular combustion chamber of the type known as the double annular type is provided with two concentric annular arrays of fuel injectors. The radially inward array is of pilot fuel injectors whereas the radially outward array is of main fuel injectors. During light up and low speed conditions, only the pilot fuel injectors are used whereas both the pilot and the main fuel injectors are used under higher speed conditions. The pilot combustion stage is long in comparison with the main combustion stage. Consequently, the residence time in the pilot stage is comparatively long, thereby limiting the emissions of hydrocarbons and carbon monoxide. The residence time in the main stage is comparatively short, thereby limiting emissions of the oxides of nitrogen.